Optical fiber amplified tapping network and method using same

ABSTRACT

An optical network includes an erbium-doped silica-based optical fibre having a D-shaped cross-section. The wave-guide carries information signals at 1.53 μm from a signal source and provides amplification to the information signals when pumped by an optical pump source operating at 0.994 μm. Optical signals are tapped from the waveguide by means of evanescent couplers. The waveguide provides amplification to at least partially restore tapping loss to the information signal due to the optical taps. The core of the optical waveguide is chosen to substantially minimize the spot size of signals at the wavelength of the pump source so as to provide preferential extraction of the information signal in order to leave the pump source substantially undisturbed to be able to pump regions of the amplifying waveguide beyond the optical tap.

This application is a 371 of PET/GB91/02052, filed Nov. 20, 1991.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical networks and in particular to opticalnetworks in which an optical waveguide of a given refractive indexprofile has a waveguiding core surrounded by a cladding to which iscoupled a source of optical information signals at a first wavelength,an optical amplifier, and a source of optical pump power at a secondwavelength shorter than the first wavelength for optically pumping theoptical amplifier.

2. Related Art

It is envisaged that optical networks will be made from various types ofoptical waveguides and optical amplifying technologies, for example,planar Si--SiO₂ waveguides, plastics or polymer based fibre guides,planar doped waveguide amplifiers and so on.

A particular example of such a network is described in the applicant'sco-pending patent application WO 90/09708 published on the 20 Aug. 1990which network is designed to provide a multiple access interconnectionscheme which can be exploited for both static and dynamic interconnectapplications. This network is based on optical fibre waveguides. Theteaching of this patent application is imported in full into thisapplication by reference.

The spatial dimension of such an interconnection network can be based onoptical D-fibres, for example. See for example Cassidy S. A. et al, 1989"Extendable Optical Interconnection Bus Fabricated using D-Fibre" IOOC1989, Kobe, Japan, Paper 21 D2-1 pp 88-89. A central component of thedesign is a multi-fibre backplane for carrying the signals and thereference channels, with discrete tapping points along its length. Thesetapping points may take the form of array connectors in the known formof crossed D-fibres. The interaction length is determined by the angleof crossover of the crossed D-fibres. Each connector provides a linkbetween each output fibre and its corresponding fibre path on thebackplane, evanescently tapping out a small portion of the power fromeach of the signal and reference channels. The limitation on the numberof the tapping points is that eventually the signal level along thebackplane will fall below detector levels (as each tap removes a smallportion of signal).

The use of erbium doped fibres, either distributed or in discreteamplifier units, allows amplification of the signal in the 1.55 μmwindow. The amplification can be arranged to maintain the output fromall the tapping points on the bus above the detector power limit. Thesignal power is regenerated by amplification between tapping points,thus allowing a significantly larger number of user ports to be served.It is convenient to distribute the pump power along the waveguide to thefibre amplifiers from a single pump source rather than provide aseparate pump source for each amplifier. However, a small amount of pumppower will also be removed at each tapping point and so wasted.

SUMMARY OF THE INVENTION

According to the present invention an optical network includes anoptical waveguide having a waveguiding core surrounded by a non-guidingregion, and a given refractive index profile, to which is coupled asource of optical information signals of a first wavelength, an opticalamplifier, and a source of optical pump power of a second wavelengthshorter than the first wavelength for optically pumping the opticalamplifier and is characterised in that the core of the optical waveguideis chosen to substantially minimise the spot size of signals at thesecond wavelength and there is a region at which the cladding issufficiently thin to allow evanescent coupling to the signals at thefirst wavelength.

The present invention is based on the realisation that if the core of awaveguide is such as to substantially minimise the spot-size at theshorter of two wavelengths, the spot size of a longer wavelength signalswill be past its minimum and be diverging from the shorter wavelengthspot size. This means that an evanescent coupler formed from such awaveguide will evanescently couple less efficiently at short wavelengthsthan at long wavelengths thereby providing a selective optical tap toselectively couple out or tap an information signal preferentially to ashorter wavelength pump signal in optical networks with optically pumpedoptical amplifiers.

Aspects of the present invention are as disclosed in the accompanyingclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings of which:

FIG. 1 is a schematic diagram of an embodiment of an optical networkaccording to the present invention;

FIG. 2 is a cross-sectional view of a D-fibre optical waveguide suitablefor use with the embodiment of FIG. 1;

FIG. 3 is a graph showing the spot size as a function of core radius,refractive index profile and signal wavelength;

FIG. 4 is a graph showing the theoretical wavelength dependence ofcross-coupled power from the waveguide of the embodiment of FIG. 1;

FIG. 5 is a graph of the spectrum analyzer plot of the actualcross-coupled power of the waveguide of FIG. 2;

FIG. 6 is a schematic diagram of a further embodiment of an opticalnetwork according to the present invention;

FIG. 7 is a graph showing the modulation of the output of a coupler ofthe embodiment of FIG. 6 as a function of pump power;

FIG. 8 is a graph of the theoretical cross-coupled power as a functionof wavelength; and

FIG. 9 is a graph of the experimentally obtained cross-coupled power asa function of wavelength.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to FIG. 1, an exemplary optical network embodying the presentinvention comprises a series of silica-based optical D-fibres 2 splicedat the positions marked "X" to interposed erbium optical fibreamplifiers 4 which collectively form an optical bus 5. A laser 6 has a1.55 μm output which is modulated with information by a laser driver 8in known manner.

The optical amplifiers 4 are pumped at 0.98 μm by a pump laser 10 whoseoutput is combined with the information signal from the laser 6 by acommercially available 980/1530 dichroic optical fibre coupler 12spliced to the optical bus 5.

The D-fibres 2 were formed by the well known method of sawing an opticalfibre preform to form a longitudinal flat along the preform and thendrawing the optical fibre down to the required size in the usual manner.This forms an optical fibre 2 having a D-shaped cross-section (see FIG.2) with a waveguiding core 22 surrounded by a non-waveguiding cladding24 having a flat surface 26 d μm from the core 22.

A similarly formed optical D-fibre 28 when placed with its flat surfaceclose to the flat surface of the optical fibre 2, as shown in FIG. 1,will be able to couple out an optical signal propagating along the core22 of the fibre 2 if the spot size of the optical signal extendssufficiently far from the core 22. The amount of that signal tapped outof the core 22 of the fibre 2 will depend, amongst other things, on thevalue of d, the interaction length between the fibres 2 and 28.

FIG. 3 shows the spot size dependence on core radius at two signalwavelengths, 1.55 μm and 0.98 μm and at two refractive index profilescharacterised by their effective step index values δn, of 0.004 and0.008. The larger the δn value the greater the differential tappingobtainable.

The design criteria for an optimum optical network shown in FIG. 1include a requirement that there is a maximum rejection of pump oversignal while maintaining a sensible interaction length with consequentlyachievable coupling tolerances and a need to maintain lowsystem-to-device splice losses at the signal wavelength. To achieve thisthe spot size of the waveguide should have an effective step index valueδn such that when the spot size at the pump wavelength is substantiallyminimised, the spot size of the signal wavelength is substantiallymatched to that of the system waveguide. Achieving this also ensuresthat the bend sensitivity of the waveguide is not increased.

From FIG. 3 it can be seen that for δn=0.004 the fibre should befabricated with a core radius in the region of a 3 μm to minimise thespot size at the pump wavelength of 0.98 μm. This core size, however,results in a mismatch of spot size at the signal wavelength between thefibre and the 5, 8 μm spot size of a standard telecommunications fibreto which the waveguide, in this example, is to be spliced.

A fibre having a δn over 0.008, however, will have the minimum spot sizeat the pump wavelength for a core radius of about 1.3 μm which alsoprovides a match of the spot size of the signal wavelength to that ofthe standard telecommunications fibre.

The preform can then be altered, if necessary, to ensure that the pulledfibre has both the required core radius and a cladding diameter of 125μm to ease splicing to standard system fibre.

The graph at FIG. 3 indicates the spot size in μm necessary to provideminimum splice loss to a standard telecommunications fibre, namely 5, 8μm. The FIG. 3 graph indicates that it is possible to operate in aregime where the spot size at 1.55 μm has passed its minimum value andis diverging rapidly with decreasing wavelength, while that at 0.98 μmis close to its minimum value. Evanescent coupling will thus take placemore strongly at the longer wavelength producing a wavelength selective(dichroic) tap. It is worth noting that the variation in cross-coupledpower for the dichroic tap is less than 0.5 dB over the 1.55 μm window.Where amplification is not required, the tap can be designed to be broadband over both 1.3 μm and 1.55 μm windows.

The degree of selectivity is dependent on the refractive indexdifference, with a higher index difference giving a highly selective tapwhile a low δn gives a broad band tap.

For any particular D-fibre geometry, the wavelength dependence ofcross-coupled power increases with core separation, as does theinteraction length required for a given level of coupling. It istherefore possible to trade increased rejection of shorter wavelengthsfor increased interaction length, by choosing the appropriate d-value(effectively the core to core separation). Providing a longerinteraction length requires the two D-fibres to cross at a smallerangle. This makes the tap more susceptible to small errors in alignmentand manufacture, and hence tighter tolerances would be required toachieve the same level of coupling.

Computer simulation of the coupling between croseed D-fibres of variousd-values allows the wavelength dependence of the tap to be predicted.FIG. 4 shows the theoretical wavelength dependence of a 10% tap ford-values of 1 μm, 2 μm and 3 μm. The interaction lengths required forthis level of coupling are 150 μm, 250 μm and 420 μm respectively. Thisis comparable to the lengths reported for previous demountable taps andhence the manufacturing tolerances required are of the same order. Thepredicted rejection of 0.98 μm pump over signal at 1.55 μm is 13 dB ford=2 μm. A tap was fabricated from a length of D-fibre of δn=0.0067 andcore radius 2.2 μm. The d-value was 2.1 μm. The coupling ratio of 10%was easily achieved, indicating that the required manufacturingtolerances were met.

FIG. 5 shows the spectrum analyzer plot for the cross-coupled leg of thetap. The through-loss of the bus fibre, with system fibre tails, showedno wavelength dependence. The wavelength dependence of the cross-coupledpower follows the theoretical curve for the longer wavelengths.Departures from the theory occur at wavelengths below 1.1 μm. Furtherinvestigation showed that a sharp fall off in receiver sensitivityoccurs at these wavelengths for powers below -80 dBm. Direct measurementof the relative powers coupled for source wavelengths of 0.98 μm and1.54 μm showed a rejection of 10.7 dB, which is in good agreement withthe theoretical value. Using taps of the cross D-fibre type describedreported here, with a signal tapping coefficient of 10%, 80 ports couldbe connected to a bus backplane before -3 dB of pump power was lost.This rejection could be further increased at the expense of using alonger device interaction length.

Referring now to FIG. 6 there is shown an experimental arrangement usedfor determining the characteristics of a network according to thepresent invention which is the same as the FIG. 1 embodiment except thatthe D-fibres 2 and erbium fibre amplifiers 4 are all formed from asingle, erbium-doped D-fibre rather than being separate erbium fibreamplifiers spliced between sections of non-amplifying D-fibre. In thisarrangement there are two taps, 32 and 34, on a waveguide 30.

The dopant density of the erbium doped D-fibre 30 of the arrangement ofFIG. 6 had a dopant density of 5.5×10¹⁸ ions/cc. The output from eachtap 32, 34 and the waveguide 30 output were power monitored. Thedistance between the two tapping points 32, 34 was approximately 10 cm.a semiconductor diode laser 36 fabricated by British TelecomLaboratories operating at a wavelength of 994 nm giving an input powerlevel on the waveguide 30, or spine, of 1.3 mW provided the pump powerfor the amplifying waveguide 30.

A signal wavelength was supplied by a DFB laser 38 operating at awavelength of 1.53 μm and giving an output power level of about 1.53 μW.In the unpumped state, the tap ratio at each coupler 32, 34 was set at7% (-11.5 dB). With the pump laser off, the power level of the output ofthe second tap 34 was 0.65 dB lower than at the first tap 32. This wasdue to the power removed at the first tap 32 (0.3 dB) and doped fibreabsorption of 0.35 dB between the taps 32 and 34. When the pump laser 36was subsequently switched on and off, the modulations superimposed onthe power output from the second tap 34 was seen to be 0.65 dB, ie thepower levels were equal at the two tap outputs while the pump laser 36was on.

Referring now to FIG. 7 there is shown the signal power level at thesecond tap 34 of FIG. 6 as a function of time. A signal modulation, dueto the pump laser 36 being switched on and off, is clearly visible. Thecorresponding modulation at the first tap 32 was barely discernablehaving a peak to peak variation of about 0.03 dB.

Measurement of the cross-coupled power levels at 0.994 μm and 1.53 μmgave a rejection value of pump over signal of 34.5 dB. The loss of pumppower by this mechanism is therefore negligible.

A variety of amplification strategies and regimes may be considered. Asimplified doped fibre analysis has been developed to aid understandingof the options available. One attractive option is to make the mostefficient use of the pump power available by optimising dopant levels tomaximise tap number and to meet a simple constraint--the spine power atthe first and final tap of a network should be equal. The maximum numberof taps will be served with a dopant level if it is such as to produce asmall net gain over approximately the first half of the bus, becoming anet loss further along the spine as the pump power is absorbed.

Although the differential spot size of signal and pump optical signalprovides differential tapping values at the two wavelengths, furtherflexibility in system design of an optical network utilizing the presentinvention may be obtained by tailoring the wavelength characteristics ofthe taps to suit a particular application. Increased wavelengthselectivity can be obtained by using non-identical D-fibres. With such acoupler it is known that 100% coupling between the coupled waveguides isonly possible when the difference in propagation constants for thecoupled wavelength is zero and the interaction length of the coupler isequal to that required for complete power transfer from one waveguide tothe other.

From computer modelling of an optical tap response, the D-fibre can bedesigned to give a chosen wavelength tapping characteristic. FIG. 8shows the predicted cross-coupled power spectrum for coupling betweenD-fibres of δn=0.004 and δn=0.0105 for an interaction length optimisedfor 1.3 μm.

For this pair of fibres the difference in propagation constants wasequal to zero at 1.23 μm and hence 100% coupling at 1.3 μm is notachieved. The experimental plot for the same pairs of fibres is shown inFIG. 9 and corresponds closely with the theoretical prediction shown inFIG. 8. A change of waveguides such that the difference in propagationconstant was equal to zero at a value of around 1.53 μm (by changing δnof one of the fibres, for example, will produce a similar narrow bandtap for the 1.55 μm window). This would be ideal for a dual wavelengthapplication as it would allow taps to be attached to the waveguide withconnection being made over a chosen wavelength band.

We claim:
 1. An optical network including:an optical waveguide having awaveguiding core surrounded by a non-guiding region, and a givenrefractive index profile, to which is coupled a source of opticalinformation signals of a first wavelength, an optical amplifier, and asource of optical pump power of a second wavelength shorter than thefirst wavelength for optically pumping the optical amplifier, the coreof the optical waveguide being chosen to substantially minimise the spotsize of signals at the second wavelength, a plurality of evanescentoptical couplers coupled with said optical waveguide, each of saidcouplers comprising a region at which the cladding of the waveguide issufficiently thin to allow evanescent coupling to the signals at thefirst wavelength whereby a portion of said signals is removed foroutput, and said amplifier being adapted to amplify the remainingsignals at the first wavelength passing along said waveguide after saidremoval to compensate for the attenuation caused by said removal.
 2. Anetwork as in claim 1 in which the optical amplifier comprises anoptical fibre amplifier.
 3. A network as in claim 2 in which the spotsize of the information signal of a first wavelength is substantiallythe same in the optical waveguide as in the optical fibre amplifier. 4.A network as in claim 1 in which the optical waveguide is doped with anactive medium and constitutes the optical amplifier.
 5. A network as inclaim 1 in which the optical waveguide is spliced to a further opticalfibre and the spot size of the information signal is substantially thesame in the optical waveguide and the further optical fibre.
 6. Anoptical network as in claim 1 in which the optical waveguide is anoptical fibre having a D-shaped cross-section.
 7. An optical network asin claim 1 in which the optical amplifier comprises a rare earth dopedsilica-based optical fibre.
 8. An optical network as in claim 7 in whichthe rare earth comprises erbium.
 9. An optical network as in claim 1 inwhich the first wavelength is approximately 1.55 μm and the secondwavelength is approximately 0.98 μm.
 10. An optical network as in claim1 in which the couplers comprise crossed optical fibres each having aD-shaped cross-section.
 11. An optical network as in claim 1 in whichthe coupler is wavelength selective.
 12. An optical network as in claim11 in which the optical waveguides comprising the optical coupler havedifferent propagation constants.
 13. An optical device whichcomprises:(a) an optical signal generator for providing a first opticalsignal at a first wavelength; (b) a plurality of output ports, and (c)an optical network interconnecting (a) with all of (b); wherein (c)comprises:(c)(i) an optical fibre waveguide connected to (a) and all of(b), (c)(ii) an optical fibre amplifier included in (c)(i), (c)(iii) anoptical pump for providing optical power at a second wavelength adaptedto drive (c)(ii), and (c)(iv) a plurality of wavelength selectiveoptical couplers each of which couples one of the (b) to (c)(i); wherein(c)(ii) is adapted to amplify said first signals between each pair ofoptical couplers so as to compensate for the power removed by theprevious coupler.
 14. An optical device according to claim 13, whichalso comprises a laser driver for modulating the output of (a) whereby amodulated signal is provided to all of the output ports.
 15. An opticaldevice which comprises:(a) an optical signal generator for providingoptical signals at a first wavelength; (b) a laser driver forcontrolling (a) so as to produce modulated signals therefrom; (c) aplurality of output ports, and (d) an optical network interconnecting(a) with all of (b); wherein (d) comprises:(d)(i) an optical fibrewaveguide containing a lasing dopant for amplifying optical signals atsaid first wavelength, (d)(ii) a pump connected to (d)(i) for providingoptical signals at a second wavelength adapted for interaction with thelasing dopant of (d)(i) so as to provide said amplification, (d)(iii) aseries of wavelength selective optical couplers spaced along (d)(i) eachcoupler being adapted to provide modulated signals to one of said outputports; wherein each of said couplers provides a portion of signals atthe first wavelength to its output port allowing the remainder tocontinue in (d)(i) and to provide substantially all of said pumpradiation at said second wavelength into (d)(i) whereby the modulatedsignals are amplified between each pair of couplers to compensate forthe signal removed.
 16. An optical device which comprises:(a) an opticalsignal generator for providing optical signals at a first wavelength;(b) a laser driver for controlling (a) so as to produce modulatedsignals therefrom; (c) a plurality of output ports; and (d) an opticalnetwork interconnecting (a) with all of (c); wherein (d)comprises:(d)(i) an optical D-fibre waveguide containing a lasing dopantfor amplifying optical signals at said first wavelength, (d)(ii) a pumpconnected to (d)(i) for providing optical signals at a second wavelengthadapted for interaction with the lasing dopant of (d)(i) so as toprovide said amplification, (d)(iii) a series of wavelength selectiveoptical couplers spaced along (d)(i) each coupler comprising a D-fibrecrossing (d)(i) in a configuration adapted to provide modulated signalsto one of said output ports; wherein each of said couplers provides aportion of signals at the first wavelength to an output port allowingthe remainder to continue in (d)(i) and to provide substantially all ofsaid pump radiation at said second wavelength into (d)(i) whereby themodulated signals are amplified between each pair of couplers tocompensate for the signal removed.
 17. A method of providing an opticalsignal to a plurality of output ports wherein the output at each portcontains substantially the same modulation said output being derivedfrom a single, modulated optical signal source, which methodcomprises:providing the signals from said single source into a singleoptical waveguide and tapping the signals from said waveguide to providesaid outputs, the proportion of signal removed at each tap beingsubstantially the same, and amplifying the signals between each pair oftaps so as to compensate for the signal removed to provide the output.18. A method according to claim 17, wherein the signals provided at eachoutput have substantially the same strength.